Paleoenvironment and Organic Characterization of the Lower Cretaceous Lacustrine Source Rocks in the Erlian Basin: The Influence of Hydrothermal and Volcanic Activity on the Source Rock Quality

Lower Cretaceous lacustrine source rocks in the Erlian Basin are highly heterogeneous. It is important to assess and explain these heterogeneities for the reconstruction of paleoenvironments and the prediction of high-quality source rock distributions. In this study, well-logging, organic, and elemental geochemical data were comprehensively analyzed for the source rocks of Member 4 of the Aershan Formation (Fm) and Member 1 of the Tengger Fm in the southern Bayindulan (BNAN), southern Wulanhua (WLHs), Anan, Aer, and southern Wuliyasitai sags of the Erlian Basin. The variability in sedimentary environments, sources of organic matter of the source rocks in different sags, and the influence of hydrothermal and volcanic activity on the source rock quality in the Erlian Basin were assessed. The results reveal that the source rocks can be divided into four types of organic facies (A, B, BC, and C). Organic facies A–B present hydrogen indices (HIs) higher than 400 mg/g and are mainly composed of mudstone and thick (average thickness >50 m) dolomitic mudstone, with biomarkers characterized by a Pr/Ph ratio lower than 1.0, a gammacerane/C30 hopane (Gam/C30H) ratio higher than 0.2, and a C19 tricyclic terpane/C23 tricyclic terpane (C19/C23TT) ratio lower than 0.6. Organic facies BC–C are composed of mudstone with an HI < 400 mg/g, with biomarkers characterized by a Pr/Ph ratio higher than 0.8, a Gam/C30H ratio lower than 0.2, a C19/C23TT ratio higher than 0.6, and a sterane/hopane ratio lower than 0.4. Dolomitic mudstone belonging to organic facies A–B is mainly developed in the BNAN, WLHs, and Anan sag and is characterized by a fault-controlled distribution in the sag, a right-declined rare earth element pattern, and an enrichment in the elements of Ba, Cu, Zn, Fe, and Ni. The genesis of high HI dolomitic mudstone is associated with hydrothermal and volcanic activity because the hydrothermal fluid or hydrolysis of volcanic ash result in increasing input of reducing gas and soluble nutrient ions, thus promoting the formation of anoxic and saline Cretaceous lakes with high primary productivity.


INTRODUCTION
The symbiosis between volcanic rocks and source rocks is common in the rift petroliferous basins of eastern China, 1 but views concerning the influence of volcanic activity on organic matter enrichment in source rocks vary greatly. Numerous studies have shown that volcanic ash produced by volcanic eruptions can release large amounts of Fe, N, P, Si, Mn, and other nutrients into the water column, which is favorable for the proliferation of algae, 2 fungi, 3 or other organisms 4,5 and further promote primary productivity. 6 Several studies have also shown that soluble gases (SO 2 , H 2 S, etc.) and metal elements (Ca 2+ , Mg 2+ , etc.) released by volcanic eruption increase water reducibility 7,8 and salinity, 9,10 which are conducive to the stratification of the water column and may further promote the preservation of organic matter. 11 The gases and dust produced during intense volcanic eruptions also form sulfate aerosols and accelerate the cooling of the climate through an aerosol-cloudclimate feedback system, 12,13 which would result in mass extinctions 14,15 and thus facilitates the burial of organic matter. 11 However, some scholars believe that the benefits of volcanic ash on organic matter enrichment are not clear 16,17 and intense water turbulence or the dilution effect caused by volcanic eruption or homeochronous hydrothermal exhalation may even cease the enrichment of organic matter. 13,18 A notable tectonic magmatic event occurred in Northeast Asia in the late Mesozoic, 19 forming a series of semi-graben-type rift basins with NE-NNE trend strikes, including the Songliao, Hailar, Erlian, and East Gobi Basins in Mongolia ( Figure 1A). The Lower Cretaceous in the Erlian Basin contains multiple sets of volcanic strata ( Figure 2). However, hydrothermal sediments have only been deposited in the Baiyinchagan sag of the Chuanjing Depression in the Erlian Basin. It is still uncertain whether the Lower Cretaceous strata in the other sags of the Erlian Basin were affected by hydrothermal or volcanic activity.
The organic facies indicate a mappable subdivision of a stratigraphic unit with similar organic constituents. 20 This concept is widely used to map the distributions of possible source rocks and associated petroleum. Some studies also use organic facies to assess the depositional environment of the organic matter because its lacustrine sediment composition provides important information on paleoclimate and hydrodynamic conditions over geological periods. 8,21,22 The Erlian Basin is one of the most petroliferous basins in northeastern China, with abundant petroleum resources. The basin is characterized by multiple small sags, obvious segmentation among sags, and oil distribution controlled by sub-sags. 23 With the improvement in the exploration of the positive structural belts, the sub-sag zone, which accounts for approximately 70% of the basin area, is considered to be the main distribution area of the remaining petroleum. Reconstruction of the paleoenvironment and genesis of high-quality source rocks are vital for assessing the hydrocarbon generation potential of source rocks in various sub-sags in the basin. Previous studies on source rocks mainly focused on a single sag, with the analysis of inorganic and bulk organic geochemical characteristics, 24−30 the oil-source correlations, 28,31−34 and the origin of lacustrine dolomites. 35−37 Ding et al. 24,25,38,39 believed that source rocks in Member 1 of the Tengger Formation (Fm) in the Erlian Basin were deposited in oxic to anoxic and fresh to brackish water conditions, with abundant terrestrial organic matter input and little aquatic organic matter input. The sources and depositional environments of the source rocks were mainly determined by differences in tectonic settings. 24 However, Wei et al. 29,36,37 and Luo et al. 28 reported that the source rocks in Member 1 of the Tengger Fm and Member 4 of the Aershan Fm in the Saihan, Naoer, and Anan sags were deposited in dysoxic to anoxic and brackish to saline water environments, with biological sources mainly from bacteria and algae. The uncertainty regarding sedimentary environments and organic matter sources has significantly inhibited the prediction of source rock and the associated oil and gas distributions. Therefore, it is necessary to conduct a comparative analysis of the organic geochemistry of source rocks in multiple sags in the Erlian Basin. By integrating organic geochemical data with elemental geochemical data, our study aimed to assess the geochemical variability of the Lower Cretaceous lacustrine source rocks in the Erlian Basin and to reveal the environmental and ecological differences in different sags. In addition, the influence of the Late Mesozoic tectonic magmatic event on organic matter enrichment in the Erlian Basin can be interpreted. The results herein are significant for the reconstruction of the paleoenvironment in the Early Cretaceous and the optimization of exploration targets in the Erlian Basin.

GEOLOGICAL SETTING
The Erlian Basin is located in northeastern China, Inner Mongolia, and covers an area of approximately 100,000 km 2 . The basin is an Early Cretaceous rift basin developed on a Hercynian folded basement 40 and is not a unified catchment basin. 41 It contains 65 sags and 21 salients in the five depressions of Manite, Wunit, Ulanqab, Chuanjing, Tengger, and the three uplifts of Bayinbaolige, Sunite, and Wenduermiao ( Figure 1B). 42 Each sag is relatively independent of the others for a long time but connected for a short time. The study area consists of the Aer sag, southern Wuliyasitai (WYs) sag in the Bayinbaolige uplift, southern Bayindulan (BNAN) sag, Anan sag in the Manite depression, and southern Wulanhua (WLHs) sag in the Wenduermiao uplift. In the cross section of the depositional centers, the BNAN, Anan, and WLHs sags had a graben structure ( Figure 1C), whereas the WYs and Aer sags had a halfgraben structure. The Anan sag could be divided into the Anan anticline, the Anan sub-sag, the Hanan anticline, and the Anan slope ( Figure 1BI). 43 The BNAN sag consists of the Western slope, the BaI stepped belt, the BaII stepped belt, the BNAN sub-sag, and the Malin slope ( Figure 1BII). 44 The WLHs sag is composed of the Hongjing uplift, the Honggeer uplift, the Saiwusu uplift, the Tumuer uplift, and the WLHs sub-sag ( Figure 1BIII). 26 The WYs sag includes the eastern slope and the WYs sub-sag ( Figure 1BIV). The Aer sag comprises an eastern steep slope, a gentle western slope, a central sub-sag, and a central anticline ( Figure 1BV).
The Erlian Basin has experienced three major evolutionary stages from the Jurassic to the present ( Figure 2). The first synrift stage occurred in the Jurassic when the coal-bearing clastic rocks were deposited in the Alatanheli Group of the Lower Jurassic and the volcanic rocks formed in the Qinganling Group of the Upper Jurassic. With a thickness of 100−200 m, this set of strata only developed in some small sags and contains a range of fair−good quality source rocks. 45 The second syn-rift stage occurred in the Early Cretaceous when the alluvial, fan deltas, and offshore-lacustrine sedimentary facies developed in the Aershan Fm, and the fan deltas, offshore-lacustrine, and semideep/deep lacustrine sedimentary facies developed in the Tengger Fm. 29 This set of strata is distinguished by its considerable thickness (750−3400 m 24,28,40,46 ) with abundant        47 The Aershan Fm can be divided into four members, with lithologic assemblages mainly composed of conglomerates, sandstone, mudstone, and some overflow facies volcanic rock in the third member. 48 The Tengger Fm is mainly composed of clastic rocks and can be subdivided into Member 1 and Member 2. A set of special lithologic assemblages, including calcareous sandstone, pebbled sandstone, dolomitic/calcareous mudstone, and mudstone, developed in the lower part of Member 1 of the Tengger Fm. 48 In general, strata deposited in the second syn-rift stage constitute a complete transgressiveregressive sedimentary cycle with a coarse-fine-coarse gradation in grain sizes, in which the maximum flood surface occurs in Member 1 of the Tengger Fm. 48 The  (Table S1) and was only used for bulk geochemistry analyses, including total organic carbon (TOC) measurements and Rock−Eval (RE) pyrolysis. To ensure that the sampling depth covered the depth range of Member 1 in the Tengger Fm and Member 4 in the Aershan Fm, these samples were continuously collected from one or two typical wells in different structural zones of each sag. For example, of the 135 samples of the WLHs sag, 15 samples were collected from the L24X well in the Honggeer uplift, 13 samples were collected from the L4 well in the Hongjing uplift, 24 samples were collected from the L12X and L1 wells in the Saiwusu uplift, 48 samples were collected from the LD1 well in the WLHs sub-sag, and 35 samples were collected from the L15X and L2X wells in the Tumuer uplift.
The second set of samples consisted of 66 dolomitic mudstone, mudstone, and seven oil sand/oil samples (Table  1). These samples were collected from multiple wells in different structural zones with different sedimentary facies. The dolomitic mudstone samples were mainly collected from the special lithologic assemblages, whereas the mudstone samples were mainly collected from sections outside the special lithologic assemblages. All samples were subjected to Rock−Eval pyrolysis and aliphatic hydrocarbon gas chromatography−mass spectrometry (GC−MS) analyses. The seven oil sand/oil samples and 28 source rock samples were further subjected to aromatic hydrocarbon GC−MS analyses to investigate the environmental and ecological differences in the different sags (Table 2). Thirteen source rock samples in the second sample set were used for trace element and rare earth element (REE) analyses (Tables  3 and 4). In addition, trace and major elemental data of 10 samples in the WLHs and BNAN sags were provided by the PetroChina Huabei Oilfield Company. Trace elemental data of nine samples in the Anan sag were obtained from ref 37.
The thickness of the dolomitic mudstone was determined based on the lithological data of 108 wells in the five sags of the Erlian Basin, which were provided by PetroChina Huabei Oilfield Company.

TOC Measurement and Rock−Eval Pyrolysis.
The samples were crushed to 100 mesh for TOC measurements in a LECO CS-744 analyzer. The crushed samples were first pretreated with 5% hydrochloric acid and then burned in a high-temperature oxygen flow to generate oxides. The TOC and total sulfur contents were calculated using the generated carbon dioxide and sulfur dioxide with standard deviations within 0.1%. Rock−Eval pyrolysis was performed using an OGE-II oil and gas evaluation workstation. The crushed samples were initially heated to 300°C in an inert atmosphere, held at the temperature of 300°C for 3 min to generate free hydrocarbon (S 1 ), and then heated to 600°C at a rate of 25°C/min to generate the pyrolysis hydrocarbon (S 2 ). The hydrogen index (HI) was calculated by normalizing S 2 to the TOC. T max was defined as the temperature corresponding to the maximum generation rate of hydrocarbons in the kerogen cracking process. The relative double difference of S 2 was less than 10%, and the deviation in T max was less than 2°C .

Gas Chromatography and GC-Mass Spectrometry.
The selected core samples were rinsed with deionized water to remove the surface contaminants and then powdered to 100 mesh for GC and GC-MS analysis. The powders were extracted using chloroform in a Soxhlet apparatus for 48 h, and the extractable maltene was separated by column chromatography on silica gel into aliphatic hydrocarbons (elution with n-hexane), aromatic hydrocarbons (elution with n-hexane/dichloromethane,1:2, v/v), and polars following the procedures of the industry standard (SY/T 5119-2008).
GC analysis of the aliphatic fractions was performed using a Model HP6890N chromatography instrument equipped with an HP-5MS fused silica capillary column (60 m × 0.25 mm × 0.25 mm). The oven temperature was initially held at 50°C for 1 min, increased to 120°C at 20°C/min, increased from 120 to 310°C at 3°C/min, and held at 310°C for 15 min. Helium was used as the carrier gas at a linear rate of 1 mL/min. The distributions of isoprenoids and n-alkanes were identified in the aliphatic fraction gas chromatograms, and the relevant ratios were calculated based on peak area integrations.
GC-MS analysis of the aliphatic and aromatic fractions was performed using a Model HP6890N GC/5975IMSD instrument equipped with an HP-5MS fused silica capillary column (30 m × 0.25 mm × 0.25 mm). The GC oven temperature was initially held at 50°C for 1 min, programmed to 120°C at 20°C /min, then to 310°C at 3°C/min, and held at 310°C for 15 min. Helium was used as the carrier gas, and the samples were   <1.0%. The contents of trace and REEs were measured using the Thermo X Series II using inductively coupled plasma-mass spectrometry, with relative standard deviations <5% for trace elements and <10% for REEs.  Table S1 and shown in Figure 4. The HI of the source rock samples ranged between 214 and 809 mg/g (average 504 mg/g) in the BNAN sag, 142 and 779 mg/g (average 379 mg/g) in the WLHs sag, 116 and 867 mg/g (average 432 mg/g) in the Anan sag, and 128 and 886 mg/g (average 420 mg/g) in the Aer sag, suggesting the I−II 2 kerogen type of source rocks. In contrast, the HI of the source rock samples in the WYs sag ranged between 107 and 387 mg/g (average 243 mg/g), indicating the II 2 −III kerogen type of source rocks ( Figure 4E). Samples in the five sags had T max values ranging from 425 to 457°C, which are suggestive of the low-mature to mature thermal evolutionary stage of the source rocks. There were samples with HI > 650 mg/ g, suggesting the I kerogen type in the BaI-stepped belt, BNAN sub-sag ( Figure 4A), WLHs sub-sag, Tumuer and Honggeer uplifts ( Figure 4B), Anan and Hanan anticlines, Anan sub-sag ( Figure 4C), and Aer central anticline/sub-sag ( Figure 4D). However, most mudstone samples had HI < 400 mg/g in the Anan slope ( Figure 4C), the Aer western and eastern slopes ( Figure 4D), the Hongjing and Saiwusu uplifts ( Figure 4B), and the WYs sag ( Figure 4E). Overall, the dolomitic mudstone mostly had HI > 400 mg/g (average 568 mg/g), which is indicative of the I−II 1 kerogen type ( Figure 4F).

Dolomitic
The TOC, sulfur content, and S/C ratio of the source rock samples in Member 1 of the Tengger Fm and Member 4 of the Aershan Fm in the five sags are shown in Table S1 and Figure 5.   Figure 5E). An average S/C ratio >0.36 was found in the samples of the BaI stepped belt, the BNAN sub-sag, the Tumuer uplift, and the Honggeer uplift ( Figure 5A,B). Average S/C ratios between 0.1 and 0.36 were found for samples in the BaII stepped belt, the Anan anticline, the Aer sub-sag, the Saiwusu and Hongjing uplifts, and the WLHs sub-sag ( Figure 5A−D). In contrast, an S/C ratio <0.1 was found for samples in the Anan slope, the Aer eastern slope, and the WYs sub-sag ( Figure 5C−E). The dolomitic mudstone samples had S/C ratios in the range of 0.03−2.2 (average 0.31) ( Figure 5F).

Biomarker Distribution. 4.3.1. n-Alkanes and
Isoprenoids. The parameters of n-alkanes, isoprenoids, and the representative total ion chromatogram of the aliphatic fractions in the 64 source rock samples and seven oil sand/oil samples are shown in Table 1 and Figure 6, respectively. The medium-chain (nC 21 −nC 25 ) and long-chain (nC 27 −nC 31 ) nalkanes peaking at nC 23 , nC 25 , and nC 27 accounted for the main fraction in the total ion chromatogram of the source rock samples in the WLHs sag. The short-and medium-chain nalkanes peaking at nC 17 , nC 19 , nC 21 , and nC 23 constituted the main fraction in the total ion chromatogram of the source rocks in the BNAN, Aer, and WYs sags. In contrast, source rock samples in the Anan sag mostly displayed a short-chain n-alkanedominated total ion chromatogram with the main carbons nC 17 and nC 19 (Table 1) Prokaryote-derived hopanes are the most abundant biomarker group among terpane molecular components. C 30 H was detected as the major peak even in the mass chromatogram of sterane (m/z 217) in the source rocks of Anan ( Figure 6B). Except for samples in the Anan sag that had C 23 tricyclic terpane/C 30 hopane (C 23 TT/C 30 H) in the range of 0.07−1.59 (average 0.32), most samples in the BNAN, WLHs, Aer, and WYs sags had C 23 TT/C 30 H < 0.1. The ratios of gammacerane/ C 30 hopane (Gam/C 30 H) were higher in the BNAN, WLHs, and Anan sags (0.02−0.94, average 0.34) than in the Aer and WYs sags (0.03−0.34, average 0.11). The ratios of C 19 tricyclic terpane/C 23 tricyclic terpane (C 19 /C 23 TT) were higher in the source rock extracts of the WYs, WLHs, and Aer sags (0.12− 1.74, average 0.68) than those of the Anan and BNAN sags (0.04−0.67, average 0.19).

Aromatic Hydrocarbons.
As shown in Figure 6, obvious difference was observed in the total ion chromatogram of the aromatic hydrocarbons extracted from the source rock or oil samples in the five sags. Abundant polycyclic aromatic hydrocarbons such as triarylsteranes (TASs) were detected in the source rocks of the BNAN and WLHs sags, and abundant alkylphenanthrenes were detected in the source rocks of the Aer sag, whereas moderate or abundant alkylnaphthalenes were detected in the source rocks of the Anan and WYs sags. All samples were found to contain a high abundance of retene and a low abundance of cadalene.
Mono-, di-, and trimethylated 2-(4,8,12-trimethyltridecyl) chromans (MTTCs) compounds were identified in the m/z 121, 135, and 149 mass chromatograms of the aromatic hydrocarbon fractions. In the BNAN sag, abundant 8-methyl-MTTC (δ-MTTC) is detected in the source rock extracts and the MTTC-ratio ranges from 0.65 to 0.89 (Table 2). In contrast, δ-MTTC was not detected in the source rock extracts of the Anan and WLHs sags and the MTTC ratio ranges from 0.76 to 1. Only 5,7,8-trimethyl-MTTC (α-MTTC) was detected in the aromatic fractions of source rocks in the Saiwusu and Hongjing uplifts, the Aer sag, and the WYs sags, and the MTTC ratio was approximately 1.0.

Elemental Geochemical Characteristics.
The concentrations of the trace and major elements are listed in Table 3. The Sr/Ba ratio ranged from 0.  1 ppm), 0.85 to 7.37% (average 4.0%), respectively, which are clearly higher than those of the upper continental crust (Zn average 71 ppm, Ni average 20 ppm, and Fe average 3.5%). 51 The concentrations of REEs are listed in

Paleoenvironment of Source Rocks in Different
Sags or Structural Zones. 5.1.1. Thermal Maturity. The C 29 ββ/(ββ + αα) and C 29 αα20S/(20S + 20R) sterane ratios are commonly used maturity parameters in the immature to mature stage (R o < 0.9%) because of the isomerization reaction of sterane. Triaryl steranes tend to crack from long-chain homologues to short-chain homologues as the thermal maturity increases from the immature stage to the wet gas window stage (R o < 2.0%). 52 There are obvious positive correlations between the C 29 ββ/(ββ + αα) and C 29 αα20S/(20S + 20R) sterane ratios ( Figure 7A), C 29 ββ/(ββ + αα) and C 20 /(C 20 + C 27 ) TAS ratios ( Figure 7B), suggesting the effectiveness of the three ratios in depicting the thermal maturity of the source rocks in the five sags. The C 29 ββ/(ββ + αα) ratio ranged from 0.16 to 0.55, without reaching the equilibrium endpoint values of 0.67−0.71, which indicates that the thermal maturity of the selected samples was not beyond the bottom threshold of the oil window (R o < 1.0%).
The widely used higher plant input indicator of retene/9methylphenanthrene (retene/9-MP) 53,54 ranged from 0.05 to 10.0 but a low abundance of cadalene was detected in the samples of the BNAN sag and the WLHs sag ( Figure 6). Besides that, the retene/9-MP ratio showed an exponential decrease with the C 20 /(C 20 + C 27 ) TAS ratio in the five sags ( Figure 7C). These phenomena suggested that retene/9-MP served as a maturity indication 55 rather than a higher plant input indicator in the Erlian Basin. C 29 ββ/(ββ + αα) was weakly positively correlated with Pr/Ph ( Figure 7D) and only co-occurred with high ratios of Ph/nC 18 and β-carotane/nC max in the range of 0.15−0.4 ( Figure 7E,F), suggesting the influence of thermal maturity on the parameters including Pr/Ph, Ph/nC 18 , and βcarotane/nC max . However, some immature and low-maturity samples in the Anan sag and Aer sag had higher Pr/Ph ratios and lower Ph/nC 18 and β-carotane/nC max ratios than the samples with similar thermal maturity in the BNAN and WLHs sags ( Figure 7D−F). This indicated that the Pr/Ph, Ph/nC 18 , and βcarotane/nC max ratios were not solely controlled by thermal maturity in the five sags. As the samples were in the immature to early mature stage, thermal maturity may not have radically affected the ratios of Pr/Ph, Ph/nC 18 , and β-carotane/nC max to the extent that the original paleoenvironmental information is obscured.

Redox Conditions.
The pristane/phytane ratio can be used to determine the redox conditions of the sediments during deposition in the context of both pristine and phytane originating from phytol in the side chain of most chlorophylls. 56 During early diagenesis, a low Pr/Ph ratio (<1) is indicative of anoxic or dysoxic conditions, 56,57 whereas Pr/Ph ratios >1 suggest oxic conditions. 56 Due to the influence of specific sources of organic matter, such as halophilic archaea, 58 extremely low Pr/Ph ratios (<0.5) are often characteristic of strictly anoxic and hypersaline conditions. 59 A higher ratio of phytane/nC 18 with a lower pristane/nC 17 suggests a more reducing depositional condition, which otherwise implies more oxic conditions. 60 β-Carotane is interpreted as a specific indicator of anoxic lacustrine or highly restricted marine environments due to its sensitivity to oxygen and low potential for preservation in sediments. 61,62 There are obvious negative correlations between Pr/Ph and parameters including Ph/nC 18 and β-carotane/nC max in the source rocks of the BNAN, WLHs, Anan, and Aer sags ( Figure 8A,C), indicating that these three  Table 2. parameters are effective indicators of redox conditions. The very low Pr/Ph (<0.5) ratios and the extremely high Ph/nC 18 (>2.0) ratios, together with the poor positive correlation between Ph/ nC 18 and Pr/nC 17 ratios for samples of the BNAN sag and the Tumuer/Honggeer uplifts ( Figure 8B), suggest an additional source for phytane, such as ether lipids from the methanogenic archaea. 63,64 It is reasonable to conclude that samples of the BNAN sag and the Tumuer/Honggeer uplifts were mainly deposited under anoxic conditions as the methanogenic archaea only thrive in certain restricted anoxic or hypersaline environments. 65 The low to moderate Pr/Ph ratios (0.23−1.07) and wide range of Ph/nC 18 ratios (0.12−5.17) and β-carotane/ nC max (0.03−0.48) for samples in the WLHs sag ( Figure 8A,C) suggest a transition from anoxic conditions to dysoxic conditions. In contrast, the moderate to high Pr/Ph ratios (0.5−1.39), lower Ph/nC 18 ratios (0.14−2.61), and lower βcarotane/nC max ratios (0.01−0.23) for samples in the Anan and Aer sags suggest a transition from dysoxic to oxic conditions.
The DBT/P ratio reflects the availability of reactive sulfur for interactions with organic matter in the depositional environment. 65 The plot of DBT/P versus Pr/Ph provides a novel, convenient, and powerful method applicable over a wide maturity range for determining the depositional environments and lithologies of the source rock. 65 Samples in the BNAN and WLHs sags are plotted in the lacustrine sulfate-poor zone, whereas samples in the Anan, Aer, and WYs sags are plotted in the lacustrine sulfate-poor zone and freshwater lacustrine zone ( Figure 8D). The results indicated that low amounts of reactive sulfur were present in the source rocks of the five sags. Source rocks in the BNAN and WLHs sags were deposited under anoxic conditions, whereas source rocks in the Anan, Aer, and WYs sags were deposited under transitional conditions ranging from anoxic to oxic conditions. It is worth noting that source rocks from the BNAN and WLHs sags displayed moderate sulfur contents (0.5−2%) 65 (Figure 5A,B), which contrast with the low amounts of reactive sulfur. The detection of fine-to coarsesized pyrite crystals in the matrix of the dolomitic rocks 29,66 explains this paradox between the low DBT/P ratio and moderate sulfur content since sulfur is mainly present as sulfides rather than thiophenes in the dolomitic rocks of the BNAN and WLHs sags.
Redox-sensitive trace elements (e.g., Mo, U, V, Cr, and Mn) are widely used to decipher and reconstruct paleoenvironmental conditions. 67 Previous studies 61,62 have proposed that a V/(V + Ni) ratio <0. 46 and Ni/Co ratio <5 point to oxic conditions, 0.46 < V/(V + Ni) ratio <0.6 and 5 < Ni/Co ratio <7 point to dysoxic conditions, and a 0.6 < V/(V + Ni) ratio and 7 < Ni/Co ratio point to anoxic conditions. As shown in Table 3, samples of the five sags display V/(V + Ni) ratios ranging from 0.31 to 0.84 with a mean of 0.66, suggesting predominantly dysoxic or anoxic conditions. In contrast, the Ni/Co ratios lie between 1.5 and 13.9 with a mean of 3.86 in samples of the five sags, suggesting  Table 2. predominantly oxic conditions. This contradiction may indicate that the redox-sensitive trace elements in the five sags are affected by hydrothermal ions, as reported in the hydrothermal deposits of Yin'e Basin. 14 The Ce anomaly can serve as a redox proxy as a reducing environment generally leads to the depletion of Ce, whereas oxidizing conditions could contribute to the enrichment of Ce. 68 The positive Ce anomaly coupled with the high Pr/Ph ratios (>1.0), low Ph/nC 18 ratios (<0.3), and the absence of β-carotane in the total ion chromatogram plot for most samples in the WYs sag suggest a predominately oxic condition.

Water Salinity.
Gammacerane is thought to be formed by the reduction of tetrahymanol in bacterivorous ciliates that typically live in the chemocline or thermocline of stratified water columns. 69,70 The high gammacerane index has been widely used to indicate evaporite or hypersaline environments 64,71 as well as water column stratification. 72 The extended tricyclic terpane ratio (ETR) is indicative of the salinity and alkalinity of sedimentary water 64 because the fossil lipids of prokaryotes in saline and alkaline lakes are rich in precursors of extended tricyclic terpanes. 73 The composition and distribution of MTTCs and the MTTC ratio in sediments can be used to reflect the paleosalinity of the depositional water. 74,75 Empirically, the MTTCs of sediments from semi-saline to freshwater environments (i.e., <30‰) are characterized by a dominance of α-MTTC and a lack of δ-MTTC, with an MTTC ratio >0.7. The MTTCs of sediments from the mesosaline environments (40−120‰) were characterized by a prominent α-MTTC relative to δ-MTTC, with 0.4 < MTTC ratio < 0.7. The MTTCs of sediments from hypersaline environments (i.e., >120‰) are characterized by a prominent δ-MTTC relative to α-MTTC, with MTTC ratio < 0.4. 76,77 Triaryl steranes tend to be abundant in the immature or low-mature organic matter of saline environments. 78 The Gam/C 30 H ratio decreased with an increasing Pr/Ph ratio ( Figure 9A), indicating that the salinity in the water body is directly controlled by redox conditions. A close positive correlation between Gam/C 30 H and ETR was found ( Figure  9B), suggesting that Gam/C 30 H and ETR were effective indicators of water salinity in the five sags. In the crossplot of Pr/Ph versus MTTC ratio ( Figure 9C Table 2. sag, and the WLHs sub-sag suggest transition conditions from freshwater to saline water. The high MTTC ratio and low to moderate Gam/C 30 H ratio (<0.4), ETR (<0.6), and the absence of triaryl steranes in samples of the Saiwusu/Hongjing uplift, Aer sag, and WYs sag suggest a predominate brackish to fresh condition.
The S/C ratios can be used to distinguish freshwater (or slightly brackish) from saline phases of ancient lakes because much lower concentrations of dissolved sulfate have been found in freshwater than in saline water. 79 Sediments in saline or brackish lakes are characterized by S/C ratios >0.1, whereas freshwater sediments are characterized by S/C ratios <0.1. 79,80 The Sr concentrations and Sr/Ba ratios in sediments are sensitive indicators for the discrimination of paleosalinity. Generally, Sr/Ba ratios of <0.6, 0.6−1.0, and >1.0 are indicative of freshwater, brackish water, and saline water, respectively. 8,81 The Sr/Ba ratio showed a general positive increase with Gam/ C 30 H ( Figure 9D), suggesting the potential of the Sr/Ba ratio as an indicator of salinity. Although Sr/Ba ratio <0. 6 and Gam/ C 30 H < 0.2 were detected in the BNAN, WLHs, and Anan sag samples, most dolomitic mudstone samples in these sags had an S/C ratio >0.1 ( Figure 5F) and Gam/C 30 H > 0.2 ( Table 2). This result supports previous observations that dolomites are formed in concentrated alkaline lakes but are absent in freshwater lakes. 62,63 In the BaI-stepped belt, BNAN sub-sag, Tumuer/Honggeer uplift, WLHs sub-sag, Anan anticline, and Anan sub-sag, the detection of the S/C ratio ≥ 0.36, average thickness of dolomitic mudstone >100 m, coupled with Gam/ C 30 H > 0.4, and ETR > 0.6 suggest a predominate mesosaline or saline condition during the deposition of source rocks in Member 1 of the Tengger Fm and Member 4 of the Aershan Fm. In the BaII stepped belt, Saiwusu/Hongjing uplift, Hanan anticline, Aer central anticline/sub-sag, Aer western slope, and WYs sub-sag, the detection of an S/C ratio of 0.1 < S/C ratio < 0.36 and dolomite thickness ratio of 0 m < average thickness of dolomitic mudstone <100 m, coupled with the 0.2 < Gam/C 30 H < 0.4 and 0.4 < ETR < 0.6 suggest a predominate brackish condition during the deposition of source rocks in Member 1 of the Tengger Fm and Member 4 of the Aershan Fm. In the Anan slope, Aer eastern slope, and WYs eastern slope, the detection of S/C ratio <0.1, Gam/C 30 H < 0.2, and ETR < 0.4 suggests a predominant freshwater condition during the deposition of source rocks in Member 1 of the Tengger Fm and Member 4 of the Aershan Fm.

Sources of Organic Matter in Different Sags.
Sterane parameters have the potential to reflect the presence of primary producers in lacustrine systems. 82 C 28 regular steranes in Cenozoic sediments are thought to be associated with specific phytoplankton types, such as diatoms 64,83 that contain chlorophyll-c, 84 whereas C 28 regular steranes in pre-Cenozoic strata are ascribed to the elevated abundance of prasionphyte green algae. 85,86 In addition, higher plants are also precursors of C 28 steranes. 87 C 29 regular steranes originate from terrestrial organic matter 87 and freshwater microalgae. 88 The 4-methylsteranes are believed to be associated with dinoflagellates that are commonly found in freshwater environments. 89 The relative abundance of hopanes is suggested to indicate the contribution of bacterial biomass to the organic matter, 90 and the S/H ratio can effectively reflect the input of eukaryotic (mainly algae and higher plants) versus prokaryotic (bacteria) organisms. 82 Tricyclic terpane hydrocarbons have been identified as major biomarkers in tasmanite algae, which are a type of green algae with close biological affinities to the present-day marine organism Pachysphaera pelagica. 91 Some researchers have also concluded that bacteria 92 and land plants 93 are likely precursors of tricyclic terpane. Tetracyclic terpanes seem to indicate terrigenous organic matter input in lacustrine source rocks and oils. 94 High ratios of C 19 /C 23 TT and C 24 Tet/C 23 TT are widely used to indicate important contributions from terrestrial organic matter, whereas high ratios of C 23 TT/C 30 H often indicate important contributions from specific microorganisms. 95 A positive correlation was found between the C 19 /C 23 TT and C 24 Tet/C 23 TT ratios of the samples ( Figure 10A), suggesting the availability of C 19 /C 23 TT and C 24 Tet/C 23 TT as indicators of terrestrial organic matter input. Positive correlations were found between the S/H and C 28 /C 29 ST ratio in the BNAN sag and the WLH sag( Figure 10B), suggesting that high S/H ratio in the saline water sags of Erlian Basin was caused by contribution from prasionphyte green algae rather than C 29 steraneproducing organisms. As the prasionphyte green algae were deposited under oxygen-depleted conditions that are unfavorable for most other photoautotrophic plankton species, 86,96 the S/H ratio may reflect the primary productivity of the source rock depositional environment. The similar C 28 /C 29 ratios but clearly lower S/H ratio in samples of the WYs, Aer, and Anan sags in comparison to that in the WLHs and BNAN sags are likely the contribution of bacteria or terrigenous organic matter.
Most source rock samples in the Anan and BNAN sags and two samples in the Aer sag have C 19 /C 23 TT ratios <0.2 and S/H ratios ranging from 0.4 to 1.7 (average 0.77) ( Figure 10C), indicating low contributions from higher plants but an important contribution from phytoplankton and metazoa in the organic matter. In contrast, most source rock samples in the WYs and Aer sags have S/H ratios < 0.4 and C 19 /C 23 TT ratios ranging from 0.2 to 1.41 (average 0.62) (Figure 10C), indicating a low contribution from aquatic organisms but an important contribution from higher plants in terms of organic matter. Source rock samples in the WLHs sag simultaneously had high S/H and C 19 /C 23 TT ratios ( Figure 10C), which suggests a mixed input of aquatic and terrestrial organisms in the organic matter. The high S/H ratio in the source rocks of the Tumuer and Honggeer uplifts, which are proximal to the boundary  Table 2. normal fault, suggests high primary productivity of the depositional water during the formation of source rocks in the footwall blocks of the boundary normal faults in the WLHs sag.

Organic Facies.
In the BNAN and WLHs sags, there were negative correlations between HI and Pr/Ph ( Figure 11A), HI and C 19 /C 23 TT ratios ( Figure 11B), and positive correlations between HI and the S/H ratios ( Figure 11C). HI > 650 mg HC/g TOC always occurred in samples with Ph/nC 18 > 1.5 ( Figure 11D). These characteristics indicate that a high HI is controlled by reductive conditions and the amount of aquatic organic matter input during the depositional process of source rocks in the BNAN and WLHs sags. In the Aer and Anan sags, there were poor relationships between HI and the parameters, including the Pr/Ph and S/H ratios ( Figure 11A,C), as seven samples with HI > 650 mg HC/g TOC had Pr/Ph ratios ranging from 0.7 to 1.1 and S/H ratios <0.4. Coincidentally, these seven samples have T max values >445°C (Table 1) and have been detected C 30 H as the major peak in the mass chromatogram of sterane (m/z 217) ( Figure 6B). These characteristics indicate that a high HI is likely controlled by an increase in bacterial input. All samples in the five sags displayed a positive correlation between the S/H ratio and ETR (Figure 10D), suggesting a close relationship between biological inputs and water salinity conditions. Specifically, an increase in lake water salinity may promote the proliferation of prasionphyte green algae and further increase water reducibility, thus contributing to the formation of high-quality source rocks.
Based on the HI and the correlations between HI and the selected parameters, including the Pr/Ph, Ph/nC 18 , S/H, and C 19 /C 23 TT ratios, four types of organic facies 20,97 can be recognized (Table 5). Organic facies A has HI > 650 mg HC/g TOC and an S/C ratio≥0.36 and is mainly composed of thick dolomitic mudstone (average > 100 m) and mudstone. Source rocks in organic facies A were deposited in anoxic environments, with abundant input from aquatic organisms or bacteria and limited input from terrestrial higher plants. Owing to the difference in thermal maturity of the organic matter, organic facies A could be further divided into organic facies A1 and A2. Organic facies A1 had T max <445°C, with the biomarkers characterized by Pr/Ph ratios of <0.5, Ph/nC 18 ratios of >1.5, βcarotane/nC max ratios of >0.2, average S/H ratios of >1.0, and C 19 /C 23 TT ratios of <0.2. Organic facies A2 had T max ≥445°C, with the biomarkers characterized by 0.5 < Pr/Ph < 1, Ph/nC 18 < 0.5, β-carotane/nC max < 0.1, and C 19 /C 23 TT < 0.2. Organic facies B had 400 < HI < 650 mg HC/g TOC, 0.1 < S/C ≤ 0.36 and was composed of mudstone intercalated with thin layers of dolomitic mudstone (average < 50 m). Source rocks in organic facies B were deposited in dysoxic environments, with a medium amount of aquatic organic matter input and a little-medium amount of terrestrial organic matter input. Biomarkers in organic facies B are characterized by 0.5 < Pr/Ph < 1, 0.4 < Ph/ nC 18 , 0.1 < β-carotane/nC max < 0.2, 0.2 < average ratio of C 19 / C 23 TT < 0.6, and 0.4 < average ratio of S/H < 1. Organic facies BC has 200 < HI < 400 mg HC/g TOC, 0.1 ≤ S/C < 0.36 and is mainly composed of mudstone that was deposited in dysoxic environments, with limited amounts of aquatic organic matter input and a medium amount of terrestrial organic matter input. Biomarkers in organic facies BC are characterized by 0.8 < Pr/ Ph < 0, 0.3 < Ph/nC 18 < 0.4, β-carotane/nC max < 0.1, 0.6 < average ratio of C 19 /C 23 TT < 1.0, and an average ratio of S/H < 0.4. Organic facies C has HI < 200 mg HC/g TOC and S/C < 0.1 and is mainly composed of mudstone deposited in oxic and fresh environments, with abundant terrestrial organic matter input. Biomarkers in organic facies C are characterized by 1.0 < Pr/Ph, Ph/nC 18 < 0.3, average ratio of Gam/C 30 H < 0.1, 1 < average ratio of C 19 /C 23 TT, and an average ratio of S/H < 0.4. Organic facies A1 was mainly distributed in the footwall blocks of boundary normal faults, such as the BaI-stepped belt, BNAN sub-sag, and Tumuer and Honggeer uplifts, whereas organic facies A2 was mainly distributed in the Anan sub-sag and Anan anticline. Organic facies B is distributed in the BaII stepped belt, Anan anticline, Anan sub-sag, WLHs sub-sag, and Aer sub-sag. Organic facies BC was widely distributed in all five sags in the  98 found that the dolomites developed proximally to the fault zones, with the thickness decreasing from the faults toward the center of the basin. Chen et al. 100 and Xiang et al. 101 identified that the hydrothermal dolomites in the Yin'e Basin and Bayan Gebi Basin have enriched Fe and Mn element contents, medium to strong negative δEu anomalies, and right-declined REE distributions. Hydrothermal and volcanic activity often occurs simultaneously. 13 In the Anan and BNAN sags, baseline research of dolomites has shown that the micro-silty dolomites in the dolomitic mudstone are formed at temperatures between 52 and 75°C, and the ion of Mg 2+ for the formation of dolomites either originates from alteration of tuffaceous materials or from the deep fluid rich in tuffaceous materials. 29,35,66,102 All of these findings suggest that the formation of dolomites in the Erlian Basin is associated with hydrothermal and/or volcanic activity.
In the BNAN, WLHs, and Anan sags, dolomitic mudstone in organic facies A−B ( Figure 4F) is characterized by an uneven distribution, with the thickness of dolomitic mudstone decreasing in the order of the footwall blocks of boundary normal faults > the sub-sag zones > the slope zones opposite to the boundary faults ( Figure 3). These characteristics suggest that the distribution of dolomitic mudstone is controlled by boundary faults similar to those of the Baiyinchagan sag. The ternary diagrams of Co−Zn−Ni and Fe−Mn−(Cu + Co + Ni) × 10 ( Figure 12) both show that source rocks in the BNAN, WLHs, and Anan sags are plot in the hydrothermal sediment zone and the hydrogenous sediment zone, whereas source rocks in the Aer and WYs sags are mostly plot in the hydrogenous sediment zone. The two diagrams objectively illustrate that the source rocks in the BNAN, WLHs, and Anan sags are lacustrine sediments interbedded or mixed with hydrothermal sediments, unlike the normal lacustrine sediments in the Aer and WYs sags. Normalization with respect to chondrites showed a similar gentle right-declined REE concentration and a similar significantly negative Eu anomaly in all samples ( Figure 13).
These features are consistent with the REE geochemical features of lacustrine hydrothermal deposits in the Yin'e Basin, 8 Bayan Gebi Basin, 101 Jiuquan Basin, 103 and Lake Tanganyika in East Africa, 104 which might indicate that the hydrothermal fluids were mixed and cooled by the lake water. 103 The lower concentration of ∑REE in the samples of the BNAN sag ( Figure  13) indicates higher alkalinity of the depositional water, as the ∑REE content was reported to decrease with higher fluid pH, 105 and may further suggest the presence of stronger  hydrothermal activity in the BNAN sag. Additionally, the higher geothermal gradient in the Tumuer and Honggeer uplifts than in the Saiwusu and Hongjing uplifts 106 also supports the presence of volcanic and/or hydrothermal activity in the WLHs sag.

Influence of Volcanic and Hydrothermal
Activity on the Source Rock Quality. As shown in Figure 14, the dolomitic mudstone had an HI > 400 mg/g, while the mudstone had an HI < 400 mg/g in Member 1 of the Tengger Fm and Member 4 of the Aershan Fm in the LD1 well of the WLHs sag, the AM2 well of the Anan sag, and the B10 well of the BNAN sag. The S/C ratio in the mudstone of the three wells mostly ranged from 0.1 to 0.36. However, S/C ratios >0.36 were only found in the dolomitic mudstone of wells LD1 and B10, which indicates that the dolomitic mudstone was mainly deposited in anoxic and saline environments. The elemental composition of the dolomitic mudstone in the LD1 well displayed a relatively higher content of nutrients (Ba, Cu, Zn, and Ni) 67,107,108 than that of the mudstone and much higher than the average values (Ba = 550 ppm, Cu = 25 ppm, Zn = 71 ppm, and Ni = 20 ppm) of the upper continental crust. 51 These characteristics indicate that the primary productivity in the depositional water of the dolomitic mudstone was higher than that of mudstone. As the formation and distribution of dolomites are associated with volcanic and hydrothermal activity, anoxic and saline environments with high primary productivity are likely the result of volcanic and hydrothermal activity. Hydrothermal fluid and hydrolysis of volcanic ash may bring a large amount of reducing gas (SO 2 , H 2 S, etc.), soluble ionic compounds, and nutrients into lakes, which is beneficial for increasing anoxic and saline conditions, thus promoting the growth and reproduction of algae and bacteria. 8 5.5. Depositional Models of Source Rocks in Different Sags. During the deposition of Member 1 of the Tengger Fm and Member 4 of the Aershan Fm in the Erlian Basin, the climate was warm and humid, 109 resulting in abundant rainfall, shortrapid river flow, and weak evaporation. This type of paleoclimate is not conducive to the formation of saline lakes. However, the extensive tectonic magmatic event resulted in an active volcanic eruption, forming multiple sets of volcanic rocks in the Lower Cretaceous and Upper Jurassic. The intensity of volcanic activity varied greatly in different sags, which is indicated by the varying content of tuff and dolomitic mudstone in the Lower Cretaceous in different sags. 41 The varying intensity of volcanic activity may have led to significant differences in the chemical properties of the depositional water in the diverse sags of the Erlian Basin.
In the BNAN, WLHs, and Anan sags of the Lower Cretaceous, there was intensive hydrothermal and volcanic activity, causing high contents of tuff, basalt, or andesite in Member 3 of the Aershan Fm and the Xing'anling Group (Figure 15a). 41,66,110,111 During the deposition of Member 1 of the Tengger Fm and Member 4 of the Aershan Fm, the BaI fault and the eastern boundary fault in the BNAN sag, the Aershan fault in the Anan sag, and the eastern boundary fault of the WLHs sag were important channels of hydrothermal and volcanic activity. 37,66 Hydrothermal fluid rich in Ca, Mg, Cu, Zn, and Ni in the deep sag surged up along the faults and mixed with the lake water. This process increased the water salinity and reducibility and was beneficial for the growth of plankton and bacteria. 112 A clear evidence of the hydrothermal activity effect in the BNAN, WLHs, and Anan sags is the distribution of dolomitic mudstone on the descending side of the boundary faults, and the high HI, S/C, C 28 /C 29 sterane, and S/H ratios in the dolomitic mudstone. At the slope opposite the faults, due to river injection and terrestrial organic matter input, the reducibility and paleoproductivity of the surface water were reduced. As a result, thick dolomitic mudstone in organic facies A and thin dolomitic mudstone in organic facies B were mainly deposited in the subsag zone and footwall blocks of the boundary faults. The organic facies BC/C were mainly distributed in the slope zones opposite the boundary fault (Figure 15a).
In the Aer Sag, the volcanic activity in the Lower Cretaceous was not intense, and no tuff or dolomites were deposited. 113 During the deposition of Member 1 of the Tengger Fm and Member 4 of the Aershan Fm, the border faults had intensive structural activity, resulting in a narrow width (<10 km) but rapid subsidence of the sag. The low width/depth ratio further hampered efficient water mixing and favored stable water column stratification, which was corroborated by the medium gammacerane abundance and the two distinct correlation coefficients between the TOC and the S content (0.1 and 0.36, respectively, Figure 5D). In the deep lake, the water was suboxic and brackish with medium to high levels of primary productivity. This can be corroborated by the S/C ratio ranging from 0.1 to 0.36, S/H ratio >0.8 and occurrence of 4methylsteranes in the source rocks of the Aer central sub-sag ( Figure 6). Thus, the source rocks in the central lake were mainly composed of organic facies B. At the edge of the sag, source rocks were deposited in shallow and oxic environments with abundant terrestrial organic matter input, mainly organic facies BC/C (Figure 15b).
In the WYs sag, the intensity of volcanic activity in the Lower Cretaceous was not intense, and no tuff or dolomite was deposited (Figure 15c). 114 During the deposition of Member 1 of the Tengger Fm and Member 4 of the Aershan Fm, the sag had a wider width (15 km) than that of the Aer sag. The high width/depth ratio was not efficient in hampering water mixing. Coupled with the abundant terrigenous clastic input, the source rocks were mainly deposited in fresh and oxic environments. Primary productivity was also reduced by the dilution effect of the large influx of terrigenous debris. Hence, the source rocks in the WYs sag consisted mainly of organic facies BC or C, which can be substantiated by the low gammacerane abundance, low S/C ratio, and HI < 400 mg HC/TOC.

CONCLUSIONS
Through comprehensive and comparative study of organic geochemical data and elemental geochemical data for source rocks in the Member 1 of Tengger Fm and Member 4 of Aershan Fm of the BNAN, WLHs, Anan, Aer, and WYs sags, the following conclusions can be drawn: (1) The source rocks in the BNAN, WLHs, and Anan sags are composed of mudstone and dolomite mudstone, which are mainly with organic facies A-BC and are characterized by high ratios of HI, S/C, and Gam/C 30 H and low ratios of Pr/Ph. The source rocks in Aer sag are composed of mudstones, which are mainly with organic facies B-BC and are characterized by medium ratios of HI, S/C, Gam/ C 30 H, Pr/Ph, and C 19 /C 23 TT. The source rocks in WY sag are composed of mudstone with organic facies BC-C and are characterized by low ratios of HI, S/C, and Gam/ C 30 H and high ratios of Pr/Ph and C 19 /C 23 TT. (2) Dolomitic mudstone is mainly distributed in the footwall blocks of boundary normal faults or in the sub-sag zone and is deposited in anoxic to dysoxic and saline to brackish environments, with biological sources originating from algae or bacteria and limited amounts of terrestrial organic matter. Mudstone with organic facies BC−C is widely distributed in the slope areas and is deposited in dysoxic to oxic and brackish to fresh environments, with biological sources originating from terrestrial organic matter and limited aquatic organic matter.